1、Designation: E 376 06Standard Practice forMeasuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Examination Methods1This standard is issued under the fixed designation E 376; the number immediately following the designation indicates the year oforiginal adoption or, in the
2、case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This p
3、ractice covers the use of magnetic- and eddy-current-type thickness instruments (gages) for nondestructivethickness measurement of a coating on a metal substrate.1.2 More specific uses of these instruments are covered byPractice D 7091 and the following test methods issued byASTM: Test Methods B 244
4、, B 499, B 530, and G 12.1.3 The values stated in SI units are to be regarded asstandard. The inch-pound units in parentheses are for informa-tion only and may be approximate.1.4 Measurements made in accordance with this practicewill be in compliance with the requirements of ISO 2178 asprinted in 19
5、82.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Reference
6、d Documents2.1 ASTM Standards:B 244 Test Method for Measurement of Thickness of An-odic Coatings on Aluminum and of Other NonconductiveCoatings on Nonmagnetic Basis Metals with Eddy-CurrentInstrumentsB 499 Test Method for Measurement of Coating Thick-nesses by the Magnetic Method: Nonmagnetic Coatin
7、gs onMagnetic Basis MetalsB 530 Test Method for Measurement of Coating Thick-nesses by the Magnetic Method: Electrodeposited NickelCoatings on Magnetic and Nonmagnetic SubstratesD 7091 Practice for Nondestructive Measurement of DryFilm Thickness of Nonmagnetic Coatings Applied toFerrous Metals and N
8、onmagnetic, Nonconductive Coat-ings Applied to Non-Ferrous MetalsE 543 Specification for Agencies Performing Nondestruc-tive TestingE 1316 Terminology for Nondestructive ExaminationsG12 Test Method for Nondestructive Measurement of FilmThickness of Pipeline Coatings on Steel2.2 ASNT Standards:2SNT-T
9、C-1A Recommended Practice for Personnel Qualifi-cation and Certification in Nondestructive TestingANSI/ASNT-CP-189 Standard for Qualification and Certi-fication of NDT Personnel2.3 AIA Standard:NAS-410 Certification and Qualification of NondestructiveTesting Personnel32.4 International Standard:ISO
10、2178 Nonmagnetic Coatings on Magnetic SubstrateMeasurement of Coating ThicknessMagneticMethod4NOTE 1See Appendix X1.3. Terminology3.1 DefinitionsDefinitions of terms relating to electro-magnetic examination are given in Terminology E 1316.4. Significance and Use4.1 GeneralNo presently available thic
11、kness gage is ap-plicable to all combinations of coating-substrate thicknessesand materials. The limitations of a particular instrument aregenerally delineated by its manufacturer.4.2 MagneticMagnetic-type gages measure either mag-netic attraction between a magnet and a coating or its substrate,or r
12、eluctance of a magnetic flux path passing through thecoating and substrate. These gages are designed to measure1This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subcommittee E07.07 onElectromagnetic Method.Current edition ap
13、proved Dec. 1, 2006. Published January 2007. Originallyapproved in 1989. Last previous edition approved in 2003 as E 376 03.2Available from TheAmerican Society for Nondestructive Testing (ASNT), P.O.Box 28518, 1711 Arlingate Ln., Columbus, OH 43228-0518.3Available from Aerospace Industries Associati
14、on of America, Inc., 1250 EyeSt., NW, Washington, DC 20005. (Replacement standard for MIL-STD-410.)4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428
15、-2959, United States.thickness of a nonmagnetic coating on a magnetic substrate.Some of them will also measure thickness of nickel coatings ona magnetic or nonmagnetic substrate.54.3 Eddy CurrentEddy-current-type thickness gages areelectronic instruments that measure variations in impedance ofan edd
16、y-current inducing coil caused by coating thicknessvariations. They can only be used if the electrical conductivityof the coating differs significantly from that of the substrate.5. Interferences5.1 Thickness of CoatingThe precision of a measurementchanges with coating thickness depending on method
17、used andinstrument design. Generally, the precision is a percentage ofthe coating thickness except at the lower end of the rangeswhere it is a fixed thickness.5.2 Magnetic Properties of Basis MetalMagnetic thick-ness gages are affected by variations of the magnetic propertiesof the basis metal. For
18、practical purposes, magnetic variationsin low-carbon AISI 1005-1020 steels may be considered to beinsignificant. To avoid the influences of severe or localized heattreatments and cold working, the instrument should be stan-dardized using a reference standard having a base metal withthe same magnetic
19、 properties as that of the test specimen or,preferably and if available, with a sample of the part to beexamined before application of the coating.5.3 Thickness of SubstrateFor each method there is aneffective depth of penetration of field created by the instrumentprobe. This is the critical depth o
20、r thickness beyond which theinstrument will no longer be affected by increase of substratethickness. Since it depends on the instrument probe andsubstrate, it should be determined experimentally.5.4 Structure and Composition of Coating and SubstrateEddy-current instruments are sensitive to variation
21、s of struc-ture, composition, and other factors affecting electrical con-ductivity and magnetic permeability of the coating andsubstrate. For example, such instruments are sensitive todifferences between: (1) aluminum alloys, (2) chromium coat-ings deposited at different temperatures, and (3) organi
22、ccoatings containing variable amounts of metallic pigments.5.5 Edge EffectAll examination methods are sensitive toabrupt surface changes of test specimens; therefore, measure-ments made too near an edge or inside corner will not be validunless the instrument is specifically standardized for such ame
23、asurement. The effect usually extends 3 to 13 mm 18 to12in. from the discontinuity, depending on method probe con-figuration, and instrument. Edge effect is usually a function ofcoil diameter.5.6 Curvature of Examination SurfaceThickness mea-surements are sensitive to curvature of the specimen. This
24、sensitivity varies considerably between instruments and be-comes more pronounced with increasing curvature.5.7 Smoothness of Surface, Including That of Base MetalSince a rough surface may make single measurements inaccu-rate, a greater number of measurements will provide anaverage value that is more
25、 truly representative of the overallcoating thickness. Roughness also may cause certain instru-ments to read high since their probes may rest on peaks.5.8 Direction of Rolling of Base MetalInstruments withtwo pole pieces may be sensitive to direction of rolling of thebase metal; that is, gage readin
26、gs may change depending onalignment of pole pieces with surface of specimen or part underexamination.5.9 Residual Magnetism in Base MetalResidual magne-tism in base metal may affect readings of magnetic- andeddy-current-type instruments.5.10 Stray Magnetic FieldsStrong magnetic fields, asfrom arc we
27、lding, can seriously interfere with operations ofcertain thickness gages.5.11 Cleanness of Probe and Test SurfaceMeasurementsare sensitive to foreign material that prevents intimate contactbetween probe and coating surface.5.12 Pressure of ProbeInstrument readings can be sensi-tive to pressure with
28、which probe is applied to test surface.5.13 Probe PositionSome magnetic-type gages are sensi-tive to position of probe relative to the earth. For example,operation of gage in a horizontal or upside-down position mayrequire a new standardization or may be impossible.5.14 TemperatureEddy-current instr
29、uments may be af-fected by temperature variations.6. Basis of Application6.1 The following items are subject to contractual agree-ment between the parties using or referencing this standard.6.2 Personnel Qualification6.2.1 If specified in the contractual agreement, personnelperforming examinations t
30、o this standard shall be qualified inaccordance with a nationally or internationally recognizedNDT personnel qualification practice or standard such asANSI/ASNT-CP-189, SNT-TC-1A, NAS-410, or a similardocument and certified by the employer or certifying agency,as applicable. The practice or standard
31、 used and its applicablerevision shall be identified in the contractual agreement be-tween the using parties.6.3 Qualification of Nondestructive Testing AgenciesIfspecified in the contractual agreement, NDT agencies shall bequalified and evaluated as described in Practice E 543. Theapplicable editio
32、n of E 543 shall be specified in the contractualagreement.6.4 Procedures and TechniquesThe procedures and tech-niques to be utilized shall be as specified in the contractualagreement.6.5 Surface PreparationThe pre-examination surfacepreparation criteria shall be in accordance with 5.11 andrequiremen
33、ts specified in the contractual agreement.6.6 Timing of ExaminationThe timing of examinationshall be in accordance with the applicable contractual agree-ment.6.7 Extent of ExaminationThe extent of examination shallbe in accordance with the applicable contractual agreement.6.8 Reporting Criteria/Acce
34、ptance CriteriaReporting cri-teria for the examination results shall be in accordance withSection 9 unless otherwise specified. Since acceptance criteria5Autocatalytically deposited nickel-phosphorus alloys containing more than 8 %phosphorus are sufficiently nonmagnetic to be measured by this method
35、, as long asthe measurement is made prior to any heat treatment.E376062are not specified in this standard, they shall be specified in thecontractual agreement.6.9 Reexamination of Repaired/Reworked ItemsReexamination of repaired/reworked items is not addressed inthis standard and if required shall b
36、e specified in the contrac-tual agreement.7. Calibration and Standardization7.1 Each instrument should be calibrated in accordance withthe manufacturers instructions and standardized before use byemploying suitable thickness standards. Standardization shouldbe checked at frequent intervals during us
37、e. Attention shouldbe given to Section 5 and Section 8.7.2 Reference standards of uniform thickness are availablein either of two types, foil or coated substrate, as supplied orrecommended by the manufacturer of the instrument. There areinstances, however, where reference standards are made byother
38、than instrument manufacturers.7.2.1 Standardization Foils (Shims)Standardization foil isplaced on the surface of uncoated base metal when standard-izing the instrument. Foils are advantageous for standardizingon curved surfaces and are often more readily available than acoated standard. To prevent m
39、easurement errors due to poorcontact between foil and substrate, make sure of intimatecontact between them. Foils are subject to indentation andshould, therefore, be replaced when damaged.7.2.1.1 Nonmagnetic foils may be used to standardize mag-netic thickness gages for measurement of nonmagnetic co
40、at-ings. Nonconductive plastic foils can be used to standardizeeddy-current instruments for measurement of nonconductivecoatings.7.2.1.2 Resilient foils should not be used if there is possi-bility that the instrument probe will cause a change in thicknessreading. Use of two or more foils on top of e
41、ach other shouldbe avoided unless flexibility of thin foils is required for acurved surface.7.2.2 Coated reference standards consist of coatings ofknown thickness permanently bonded to the substrate material.7.3 Thicknesses of reference standards should bracket andbe as close as possible to the coat
42、ing thickness being measured.7.4 For magnetic instruments, reference standards shouldhave the same magnetic properties as the coated specimen.7.5 For eddy-current instruments, the reference standardshould have the same electrical and magnetic properties asthose of coated specimen being measured (see
43、 5.4).7.6 To determine standardization validity, a reading on abare specimen identical in magnetic and electrical properties tothat of the test specimen substrate is recommended.7.7 If the coating process is changed, the standardizationmay no longer be valid, especially for magnetic coatings andeddy
44、-current gages (see 5.4).7.8 In some cases, calibration of instruments with two-poleprobes should be checked with the poles rotated 0, 90, 180, and270 (see 5.8 and 5.9).7.9 The substrate thickness for examination/measurementand standardization should be the same if the depth ofpenetration referred t
45、o in 5.3 is not exceeded. Very often it ispossible to back up the substrates of standard and examinationspecimens with sufficient thickness of the same material (toexceed the critical thickness) and make readings independentof substrate thickness.67.10 If the curvature of the coating to be measured
46、is soarched as to preclude standardization on a flat surface, then thecurvature of the coated standard or of the substrate on whichthe foil is placed should have the same contour.8. Procedure8.1 Operate each instrument in accordance with the manu-facturers instructions giving appropriate attention t
47、o factorslisted in Section 5.8.2 Check the instrument calibration at the site each time theinstrument is put into service and at frequent intervals duringuse to assure proper performance.8.3 Observe the following precautions:8.3.1 Thickness of SubstrateWhen thickness of the sub-strate is less than t
48、he critical thickness (see 5.3), and cannot bebacked up by the same metal, measurements with eddy-currentgages should not be made over metal surfaces, or otherelectrically conducting materials.8.3.1.1 With magnetic gages the effective thickness of a flatsubstrate can be increased by placing it on a
49、flat layer ofmaterial of the same magnetic properties.8.3.2 Edge EffectsReadings should not be made closerthan 13 mm 12 in. from edges, holes, inside corners, etc., ofa specimen unless validity of calibration for such a measure-ment has been demonstrated (see 7.5).8.3.3 Curvature EffectsIf the instrument has been cali-brated with a specimen of similar curvature, measurement andcalibration should normally be made with the same probeorientation.8.3.4 Number of Readings:8.3.4.1 Because of normal instrument variability, it is nec-essary to make seve
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